The present invention relates to a current limiting apparatus comprising a current limiter which changes to be high resistant state to limit or to break a current when an overcurrent such as short-circuit current is passed and which returns to the normal resistant condition by self-recovery when the overcurrent is reduced to the normal current. It has especially an advantage of effective heat radiation.
Heretofore, the current limiting apparatus shown in FIGS. 1 and 2 has been used.
In FIGS. 1 and 2, the reference numeral (1) designates a current limiter; (11) and (12) designate current terminals which are independently formed at both of ends of the current limiter (1) and which have each screw thread; (2) designates cooling fins made of aluminum which has each female screw hole connected to each current terminal (11), (12) and the female screw hole is formed in the direction of grooves for the fins and a current lead-in terminal (21) is projected from the cooling fins (2). The current limiting apparatus is assembled by screwing each bolt part of the current limiter (1) into each female screw hole of the cooling fins (2) and fastening them with each clamp nut (3).
FIG. 3 shows the current limiter (1) in detail.
In FIG. 3, the reference numeral (13) designates an insulator made of magnetic shield material such as BeO and a communicating hole is formed at the center of the insulator; (11) and (12) designate current terminals which are independently disposed through each insulator (13) at each end of the current limiter and have each space at each inner part. A closed housing is formed by the current terminals (11), (12) and the insulator (13). The reference numeral (14) designates a current limiting substance which is filled in the housing and which electrically connects between the current terminals (11), (12). For example, sodium metal is used as the current limiting substance. The reference numeral (15) designates a piston which is slidably fitted in the housing so as to perform the inner pressure buffering function; (16) designates a charged gas which is compressed by sliding the piston (15) to perform the spring function; (17) designates a cylindrical tube made of non-magnetic metal which cover the current limiter (1).
The current limiting operation of the current limiting apparatus will be illustrated.
When overcurrent such as short-circuit is passed through the current limiter (1), the current limiting substance (sodium metal) (14) is suddenly converted to be plasma having high resistance at high temperature under high pressure by self-Joule heating whereby the resistance between the current terminals (11), (12) are suddenly increased to control the current.
Rates of increases of resistance of the sodium metal depending upon rising of temperature are shown in the following table.
Table ______________________________________ Rate of increase of resistance of sodium metal (at 20.degree. C. specific resistance 4.6.mu..OMEGA.cm) Rate of increase Temperature State of resistance ______________________________________ 40.degree. C. (normal) solid 1.0 98.degree. C. liquid 2.2 883.degree. C. gas 100 5000.degree. K. plasma 4800 ______________________________________
When the overcurrent is passed, the gas at high temperature under high pressure is filled in the housing of the current limiter (1). Thus, the pressure is buffered by the action of the piston (15) as a pressure buffering element.
When the overcurrent is broken, the sodium metal is cooled and is rapidly recovered to be the normal condition by the action of the piston (15) pressed by the charged gas (16) whereby the continuous current feeding is attained.
When the current limiting apparatus is used under passing the rated current, the Joule heat generated in the current limiter (1) is transmitted through the bolts parts of the current terminals (11), (12) and is radiated from the cooling fins (2) of the current terminals.
In order to increase the rated current of the current limiting apparatus,
(a) an inner resistance of the current limiter is decreased to reduce the generation of the Joule heat or
(b) the Joule heat generated in the current limiter is effectively radiated out of the system.
Accordingly, in the former case (a) the inner resistance is decreased by increasing the amount of the current limiting substance filled in the current limiter. However, the size of the current limiter is increased and the cost is increased, disadvantageously. In the latter case (b), the size of the cooling fins is increased to radiate effectively the heat. However, the size of the apparatus is increased disadvantageously. Both of the considerations are not suitable to overcome the problem.